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Anxu00

Has anyone attempted a manual descent using a different mode than VNAV?

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This plane hand-flies very well but I always seems to be in over speed when using V/S and select a descend.  I have read in the system doc that the throttles need to be managed but there is no example.  Aside from V/S mode which other mode can I use for a manual descend?

 

Thanks in advance.

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Hi Vu,

 

I've done quite a few. The basic modes are VNAV, V/S, and PITCH HOLD. You can even use all 3 interchangeably. To give a good example, last night I flew KPUW-KSEA, and was expecting 16L, and Set up VNAV accordingly. Then, low and behold....runway change and got assigned 34R. That put me in a pinch since it gave me a lot less time and distance to descend. So, I switched to V/S, set it at -2200, managed my speed by reducing the power a little more, got the chart out, etc.

 

As for managing your speed and throttle, it's all on you and the situation. There is no real set way to do it other than the very basics- need to slow down? Reduce the power. Need to speed up? Increase the power. The best way to do it is go by 5's on your torque, set the power, and leave it for a few seconds and see what it does.

 

For an example, your at FL200 and 265kts, and are going to descend at -1800 fpm.

 

- Once you begin your descent, pick an airspeed you want to hold and bug it, say 260 kts to 10000 ft.

 

- Next, pick a power setting on the torque. At 850 RPM, pick for example 25%. Once you begin your descent set your power to 25% and leave it there, and see what it does.

 

- If you find your speed is still increasing, decrease your power to 20% torque. Leave it, and see what it does.

 

- On the other hand, if you find your speed is decreasing below 260 with 25% torque, increase it to 30%, and see what it does.

 

- Once your airspeed is stabilized, then you can fine-tune it more to be more exact.

 

Keep in mind that performance changes with lower altitudes, and you will have to adjust as necessary. This applies for any mode of descent. Keep in mind though if you use PITCH HOLD that both your v/s and airspeed will change with power settings, so that will be the most difficult to manage.

 

Hope this helps!

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I've not tried it myself yet but I am pretty sure you can use IAS mode too, that will adjust pitch to keep a set speed, if you pull the PL's back to almost idle you should get a good rate of descent with a speed set at 220 to 240 or so. Guess what I am going to try now in FSX, lol.

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I've not tried it myself yet but I am pretty sure you can use IAS mode too, that will adjust pitch to keep a set speed, if you pull the PL's back to almost idle you should get a good rate of descent with a speed set at 220 to 240 or so. Guess what I am going to try now in FSX, lol.

 

You can, I totally forgot about that ^_^ .

 

For that, power will change your V/S, more power= decreased -V/S, less power= increased -V/S, and airspeed remains constant.

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Anxu00, on 29 Apr 2013 - 6:00 PM, said:

 

This plane hand-flies very well but I always seems to be in over speed when using V/S and select a descend. I have read in the system doc that the throttles need to be managed but there is no example. Aside from V/S mode which other mode can I use for a manual descend?

 

Thanks in advance.

In regard to the title of this topic...

 

Of course! It's basic piloting. Very simple in vertical speed mode. Generally speaking you will find up to -1800 feet per minute is fine. Simply throttle back accordingly, so as not too overspeed. No different to any other aircraft.

 

IAS mode is also permissible but personally I haven't tried it yet.

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Thanks all for sharing your experiences.  There were not much aircraft data about Vne etc. only two tables with take off and landing performance with various flaps setting.  It's a certainly a fun plane to fly.  

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I usually fly on Vatsim, and live ATC tends to clear you down to STAR waypoint appropriate altitudes in good time. I've rarely had to ask for descent.

I usually descend on IAS mode, having low RPM set and throttle at near idle I tend to end between 1800-2200fpm. It changes the lower you go. That's why I prefer IAS, it guarantees you keep a decent speed with the engines at idle and adjusts VS to suit. It also guards you from overspeed and stall speed. Just remember that VNE lowers the lower you go.

I have no idea if this is a correct procedure but it keeps things simple to me and I've never had any VATSIM or IVAO complaints.

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I believe most Q400 SOP's use IAS in the climb (eg... 185 after flap retraction, and 210 after 10,000'), but VS in the descent.

 

As far as I can see, a lot (possibly most) Q400s don't have VNAV, and have 'INOP' where the VNAV button would be. (This is the case with most of the FlyBE fleet).

 

So we're lucky to have it fitted to our Majestics :wink:

 

Oh and ............ Remember your ALT SEL :smile:

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i just bought the plane yesterday. i did not read anything but using common sense looked for the apu, started it and then went on to start the engines using the starter switches. did not compute init ref or even turn on the fms. just set flaps 5, added power and rotated at 110 knots. to me it seems that the controls are not as responsive as the aircraft seems to follow the motion of the yoke with a delay of maybe half a sec or more. this behaviour forced me to think ahead and i turned for final into khqm and landed at about 100 knots with the throttles halfway. so this bird does require some serious power applied while landing. landing was a breeze, un- managed, with the flaps fully applied. sorry about the lack of pictures as i have yet to master the fine art of taking screenshots. happy flying.

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I never use VNAV. Never.

 

Here's what you do. Cruise altitude - altitude you are going down to * 3.

Example: At CI06 you have to be at 2000. So, if you are crusing at FL260 then you do (26-2)*3=72. So 72 miles out.

 

Now to find out your FPM down. GS in kt / 2 * 10 = xxxx vertical speed.

Example: Cruising at 320 kts ground speed. So, 320/2=160, and 160*10 is 1600. So you would go down 1600 feet per minute during your descent.

 

 

I hope you might find these equations useful :D

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I am also used to not using VNAV. I flew the Tu-154 a lot before I switched to FSX and got used to computing my own TOD. I will just need a couple flights in the Dash to find the sweet spot... I used to be able to routinely do power off descends from cruise down to 1000ft AAL just by eyeballing the proper TOD and playing with speed a bit. I expect to get to a similar point with the Dash.

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3° flight path:

FL / 3 = dist. to start decent

ex:cursing at FL240, 240/3=80, start decent 80 nm out

 

To check during decent, if on a 3° flight path:

Dist x 3 = FL you should be at

ex: I'm 50nm from runway, 50x3=150...I should be at FL150

 

Sinkrate = half the GS x 10,

ex: my GS is 240kts, 240/2x10=1200 f/m, or 24/2=12 & just add two 00's =1200 f/m

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Not quite correct.... :Nerd: For 3° descent path:

 

FL / 3 = dist. to start decent

ex:cursing at FL240, 240/3=80, start decent 80 nm out

 

To check during decent, if on a 3° flight path:

Dist x 3 = FL you should be at

ex: I'm 50nm from runway, 50x3=150...I should be at FL150

 

Sinkrate = half the GS x 10,  ex: my GS is 240kts, 240/2x10=1200 f/m,

or 24/2=12 & just add two 00's =1200 f/m

 

 

 

You basically just re-quoted what he said? lol 

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You basically just re-quoted what he said? lol 

Really? there's a difference between FL/3 = dist. to start decent & FLx3 /10.......LOL!

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Really? there's a difference between FL/3 = dist. to start decent & FLx3 /10.......LOL!

Not quite sure what you're trying to say, you both calculate the same thing lol

 

Im at FL300 and should be at 10'000ft for an approach at a specific point, so therefore I need to calculate when to initiate my descent in order to descend 20'000ft.

 

means (20'000 /1000) * 3 =  roughly +60NM (just as Nathan said),

 

now to calculate what FPM you need by using your current airspeed:

 

Cruising at 430kts means 430/0.2 = 2150 fpm for optimal rate of descent 

 

 

 

Now with what you said;

 

FL / 3 = dist. to start decent

ex:cursing at FL240, 240/3=80, start decent 80 nm out

 

 

FL200/3 = 66NM

 

Sinkrate = half the GS x 10,  ex: my GS is 240kts, 240/2x10=1200 f/m,

or 24/2=12 & just add two 00's =1200 f/m

430/2 = 215x10 = 2150 fpm

 

 

Any thoughts?

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I have a thought...

 

They do not calculate the same thing. The flight level change / 3 method results in a distance of 66NM for an altitude change of 20,000 ft. The altitude change *3 method results in a distance of 60NM for an altitude change of 20,000ft.

 

Nathan's method is closest to the true 3 degree glidepath (it's actually a 3.14 degree slope, but pretty close). Rodd's method produces a 2.82 degree slope. While Nathan's method is closer to the true 3 degree slope, Rodd's results in a more conservative distance, giving some extra breathing room. So in the end, they both achieve the same goal, albeit slightly different numbers.

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raysalmon, on 12 May 2013 - 5:21 PM, said:

 

I have a thought...

 

They do not calculate the same thing. The flight level change / 3 method results in a distance of 66NM for an altitude change of 20,000 ft. The altitude change *3 method results in a distance of 60NM for an altitude change of 20,000ft.

 

Nathan's method is closest to the true 3 degree glidepath (it's actually a 3.14 degree slope, but pretty close). Rodd's method produces a 2.82 degree slope. While Nathan's method is closer to the true 3 degree slope, Rodd's results in a more conservative distance, giving some extra breathing room. So in the end, they both achieve the same goal, albeit slightly different numbers.

 

Exactly, and thus my statement: "You basically just re-quoted what he said? lol "

 

Both will calculate the same thing = get you close enough to where you want to be at a specific altitude,

 

you will never be spot on due to wind changes, un-even airspeeds and so on unless you are using an A/P to have the green banana calculate it for you on the go

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Exactly, and thus my statement: "You basically just re-quoted what he said? lol "

 

Both will calculate the same thing = get you close enough to where you want to be at a specific altitude,

 

you will never be spot on due to wind changes, un-even airspeeds and so on unless you are using an A/P to have the green banana calculate it for you on the go

 

Yup...6 of one half a dozen of the other.

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I've not tried it myself yet but I am pretty sure you can use IAS mode too, that will adjust pitch to keep a set speed, if you pull the PL's back to almost idle you should get a good rate of descent with a speed set at 220 to 240 or so. Guess what I am going to try now in FSX, lol.

 

So when ATC tells me to descend the best and easiest mode to use is IAS, esp when I have not set up an approach to use VNAV? Then when I get assigned a STAr and runway I can switch to VNAV for landing?

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I use the FlyBy E6b app on the iPad and it seems to do a pretty good job getting me to meet altitude restrictions whenever I need to descend, as well as takes input as to how much altitude I need to lose, average groundspeed that I estimate, as well as vertical speed suggested. I can't seem to get the hang of the FMC VNAV, but that might just be due to lack of use, I'm a little bit more comfortable doing it the manual way. And plus, it's a little bit more hands on and fun in my opinion!

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Which part of the app do you use I just bought it will try it out? Can you give an example of how you use it appreciate it megs8888.

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Hi,

 

If you use VS in descent, WATCH THE SPEED. Use of vertical speed mode has resulted in many crashes/incidents over the years, either due to stall in climb, or because the autopilot didn't capture the altitude at low level in descent. It will also allow you to overspeed the aircraft.

 

In this aircraft more than many, you must watch the ALT SEL. Every time you move that little black wheel, check the FMA, and press ALT SEL for good measure, checking the FMA again. Nearing the level-off altitude, ensure the altitude captures. The real aircraft can fail to capture more often than is liked, and the sim is no exception.

 

VNAV can be quite useful, but requires that you stay ahead of the aircraft. The Q400 is one fast turboprop, so you need to be thinking more in terms of jet speeds when you're preparing for descent. You need to be sure you prepped the FMS in advance and that you know your altitude constraints. It is great for computing ToD, but IMHO adds to the workload.

 

Because of the large props, the Q400 (like many turboprops) has the advantage that it can stay higher, longer, and descend quicker than a jet aircraft might, especially from the higher altitudes. If you're on top of things, you can comfortably use 2 x your altitude for descent distance, especialy if you let the aircraft accelerate during descent, which is particularly useful over mountainous terrain. It also means the aircraft does a pretty decent job of decelerating whilst descending, too.

 

IMHO the best mode for climb or descent is IAS. You select a speed (~160 kts for Vx, ~180/210 kts for Vy in climb, 240 kts for descent) and either open or close the power to adjust vertical speed.

 

Best regards,

Robin.

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Yes, as Robin said IAS works great in descent. I too select 160, works great. Although I believe most airlines that operate the Q400 use VS mode in descent.

 

True about the capability to decelerate rapidly too, either in descent or on approach. Q400's have been known to approach at 240 knots, and not decelerate until 6 miles out. The huge drag from those props at Disc serve as superb speed brakes.

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To use the VNAV you need to pull power off (sometimes quite sharply at TOD) to keep out of overspeed (Especially at FL250 where VNE is down around 245kts and your cruise speed 240kias.

 

possibly a good idea to pull back on speed before you go over the hill.

 

Of course the good ol' Altitude minus 3 zeroes times by 3 = Top of Decent is always a good trick. Groundspeed times 5 = decent rate.

Remember to subtract airfield elevation from the -3x3 calculation.

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